1. Field of the Invention
The invention relates to a charging apparatus suitably used in an image forming apparatus of an electrophotographic type for charging an image bearing member such as a photosensitive member.
2. Description of Related Art
In an image forming apparatus of an electrophotographic type, a corona charger has heretofore been used as charging means for a photosensitive member. In this method, however, corona products such as ozone and nitrogen oxides are produced during the generation of corona, and these adhere to the surface of the photosensitive member or deteriorate the surface of the photosensitive member. Therefore, the spread of an image or a faint image has been caused or the stains of a corona wire have caused the non-uniformity of charging to thereby cause a faulty image such as the blank area or black line of an image.
In recent years, however, owing to such merits as low ozone and low electric power, a contact charging apparatus, i.e., an apparatus of a type in which a charging member having a voltage applied thereto is brought into contact with a photosensitive member to thereby effect the charging of the photosensitive member, has been proposed, for example, in Japanese Patent Application Laid-Open No. 57-178267, Japanese Patent Application Laid-Open No. 58-40566, etc. and has been put into practical use. Specifically, charging is effected by gap discharge caused in a minute gap between a charging member and a photosensitive member by a voltage of the order of 1 to 2 kV being applied to between the charging member and the photosensitive member.
Accordingly, a voltage equal to or greater than a certain threshold value voltage is applied, whereby charging is started, and a predetermined DC voltage is applied, whereby the photosensitive member can be charged. However, in a method wherein only a DC voltage is applied, the resistance value of the contact charging member is fluctuated by the fluctuations of the temperature and humidity around an image forming apparatus and the photosensitive member is shaved by being repetitively used, whereby the film thickness thereof is changed and a charging voltage is fluctuated and therefore, it has been difficult to bring the potential of the photosensitive member to a desired value.
Therefore, in order to achieve the further uniformity of charging, use is made of a method as disclosed, for example, in Japanese Patent Application Laid-Open No. 63-149669, etc. wherein a vibration voltage comprising an AC component having a peak-to-peak voltage twice or more as great as a discharge threshold value voltage and superimposed upon a DC voltage corresponding to a desired charging voltage is applied to a contact charging member to thereby effect the charging of a photosensitive member. This is directed to the uniformizing effect of potential by the application of an AC voltage, and the potential of the photosensitive member converges into the applied DC voltage and is not affected by such extraneous factors as the environment and the shaving of the photosensitive member.
Also, in contact charging, an apparatus of a roller charging type particularly using a roller-shaped charging roller as a charging member is preferably used from the viewpoint of the safety of charging. In the contact charging apparatus of the roller charging type, an elastic roller (charging roller) having electrical conductivity as the charging member is brought into pressure contact with the photosensitive member, and a voltage is applied thereto to thereby charge the photosensitive member. As described above, by using a charging process of the so-called contact charging type in which the electrically conductive member is brought into contact with the surface of the photosensitive member, and a voltage is applied to the electrically conductive member, the production of ozone has become little and the charging by low electric power has become possible. Further, by the method of applying a vibration voltage of (direct current+alternating current) to the charging member, it has become possible to effect stable and uniform charging.
Even in such a contact charging apparatus, however, the essential charging mechanism uses a discharging phenomenon from the charging member to the photosensitive member and therefore, a slight amount of ozone is produced. Also, discharge occurs near the surface of the photosensitive member and therefore, damage to the surface of the photosensitive member by charging becomes greater than that in the case of corona charging. Further, when use is made of a method of superimposing an AC voltage for the uniformization of charging, it has posed a problem for the deterioration of the surface of the photosensitive member to become more remarkable due to a further increase in the amount of produced ozone and an increase in discharge.
Particularly, in an OPC photosensitive member using an organic photoconductor (OPC) as the photosensitive member, the damage to the surface of the photosensitive member by contact charging is remarkable and as compared with corona charging, the shaving of the photosensitive member during the repetitive use thereof in the case of contact charging is very great and accordingly, there has arisen the problem of the service life of the photosensitive member becomes short.
On the other hand, as an attempt to extent the service life of the photosensitive member, an attempt to form the surface layer of the photosensitive member by resin hardened by light or heat, and increase the surface hardness to thereby decrease the shaving is shown, for example, in Japanese Patent Application Laid-Open Nos. 51-66834, 57-207258, and 4-226469, etc.
However, when the photosensitive member having its surface layer formed by the hardenable resin, as described above, is used in an image forming apparatus effecting contact charging, the shaving is very little, but when image forming is repetitively effected under a high-temperature high-humidity environment, there has been the problem that it is difficult to shave and remove any discharge product adhering to the surface of the photosensitive member by discharge and therefore the resulting image becomes faint.